Bava Metzia 49: When is a Change an Exploitation?

Is it ever alright to renege on an agreed upon exchange, or will this always result in the rabbis's disapproval and a curse for going back on an oath in G-d's name?  The rabbis share a number of examples that introduce this question.  If a detail changes - the value of an item, the circumstances of an exchange - do we have reason enough to excuse a buyer or seller from their oath?  

The rabbis also question the nature of the agreement.  Was the oath spoken or was it written? Wouldn't the spoken agreement - uttering an oath - demonstrate one's intention? And does it truly make a difference whether someone is the buyer or the seller decides to renege?  And what counts as an 'act of bad faith' as opposed to a halachic transgression?  As much as the rabbis are eager to clarify the parameters of this argument, they recognize that the lines can be blurry when examining a changed contract.

A new Mishna teaches about how to measure an exploitation.  The Mishna tells us that one can claim exploitation when one-sixth of a transaction is allegedly taken from the seller.  This one-sixth is based on the four silver ma'a out of the twenty-four silver ma'a in a sela.  Next, the Mishna tells of Rabbi Tarfon of Lod who was celebrated by the merchants when he ruled that exploitation is one-third of the transaction.  However, when he then says that buyers have the entire day to renege, rather than the time it takes to show the item to a relative, the merchants were upset.  They reverted back to the words of the rabbis.

The Gemara reminds us that the seller can always renege.  The remainder of our daf considers whether the purchased item might have to be of a certain value.